goldfish are omnivores , they eat bot plant and animal protein. If you have pond snails or other cold water snails in with your goldfish the goldfish will eat or at least try to eat the smaller snails.If you get too many snails in your tank then you are probably overfeeding the tank. One thing that you can do in this problem is to cut back on feeding the fish and crushing the snails against the glass so that the goldfish can eat the snails, it is good protein for them. also make sure to do regular water changes and vacuum the gravel to get rid of uneaten food and excrement.

I don't think goldfish will eat the snails. In fact I don't like having the snails in the aquarium as they will eat the plants. And it seems it's hard to remove all the snails as they grow much more in a short time.
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I think goldfishes are naturally vegetarians.So there is no reason why they should eat the snails. Snails will do well even in moderate water conditions,and dont need extreme care. Ive never been able to keep snails for long, because i keep cichlids and they love to play with snails.
(If you have a pair, and there is enough space at the top, the snails will lay eggs. Its a very good sight to see the young ones coming out of the eggs into the water.)

Since snalis have very hard and strong shells, it is very unlikely that your goldfish will eat your snalis. I would recommend you to buy Apple snalis as they do very well in fresh water tanks. Good luck with your fish keeping!